But like Mike said, but in a different way, you should fill the tank. If it holds 500ml, then make that much, even if only one roll, to keep the developer from sloshing around and having possible surge issues or bubbles. Always leave a bit of room in the tank, though, for the chems to move around for agitation.
I always measure what it takes to just cover the reels with 2 reels in the tank (I don't have a single reel tank), then add a bit more - which for my system is 425ml. Ensures covering the reels, and enough space for movement. Also works for 120 film, 1 roll. I make 500, use 425, and dump the rest if it's one shot.